package com.wxg.ch04;

import com.wxg.ch03.queue.Queue;

public class LinkedListQueue<E> implements Queue<E> {

    private class Node {
        public E e;
        public Node next;

        public Node(E e, Node next) {
            this.e = e;
            this.next = next;
        }

        public Node(E e) {
            this(e, null);
        }

        public Node() {
            this(null, null);
        }

        @Override
        public String toString() {
            return e.toString();
        }
    }

    private Node head, tail;
    private int size;

    public LinkedListQueue() {
        head = null;
        tail = null;
        size = 0;
    }

    @Override
    public int getSize() {
        return size;
    }

    @Override
    public boolean isEmpty() {
        return size == 0;
    }

    @Override
    public void enqueue(E e) {
        if (tail == null) {
            tail = new Node(e);
            head = tail;
        } else {
            tail.next = new Node(e);
            tail = tail.next;
        }
        size++;
    }

    @Override
    public E dequeue() {
        if (isEmpty()) {
            throw new RuntimeException("Cannot dequeue from an empty queue!");
        }
        Node removeNode = head;
        head = head.next;
        removeNode.next = null;
        if (head == null) {
            // 此时链表为空，说明之前链表中只有一个元素了。
            // 也即 head == tail == removeNode
            // head 和 tail 都指向最后一个元素，
            // 这种情况下需要将"head"和"tail"都赋值为`null`，以彻底断开连接
            tail = null;
        }
        size--;
        return removeNode.e;
    }

    @Override
    public E getFront() {
        if (isEmpty()) {
            throw new RuntimeException("Queue is empty!");
        }
        return head.e;
    }

    @Override
    public String toString() {
        StringBuilder res = new StringBuilder();
        res.append("Queue: front ");

        Node cur = head;
        while (cur != null) {
            res.append(cur + "->");
            cur = cur.next;
        }
        res.append("NULL tail");
        return res.toString();
    }


    public static void main(String[] args) {
        LinkedListQueue<Integer> queue = new LinkedListQueue<>();
        for (int i = 0; i < 5; i++) {
            queue.enqueue(i);
            System.out.println(queue);
        }
        queue.dequeue();
        System.out.println(queue);
        queue.dequeue();
        System.out.println(queue);
    }
}
